1. Field of the Invention
The present invention pertains to applications for Direct Digital Synthesis (“DDS”), and, more particularly, the suppression of spurious DDS signals.
2. Description of the Related Art
Conventional DDS's generate discrete sidebands as spurious signals combined with their desired output frequency. These are problematic for sensor and radar applications which require sources of clean frequencies without spurious signals. This can be the limiting factor in the performance of advanced sensor and radar systems. Existing systems must either live with this performance impact or use complex, costly radio frequency (“RF”) circuits with limited frequency agility to generate critical source signals. In current DDS applications that filter the DDS output, the high spurious level requires that the output frequency pass through a complex mix, divide and filter process to reduce the spurious signal levels to a usable level. This level is more than 20 dB below the present capability of conventional DDS's.
More particularly, filtering the DDS spurious energy is possible for some very limited applications—i.e., fixed, non-adjustable frequencies. The required precision and narrowness of the needed filters is extremely challenging and greatly increases the cost of the sub-system. These restrictions normally make this approach impractical for most applications. High performance systems generally cannot use DDS's directly to generate critical signals. Indirect methods are used to generate frequencies with low spurious signals but are limited by the additional circuit complexity needed to individually produce each additional frequency used by the system.
The present invention is directed to resolving, or at least reducing, one or all of the problems mentioned above.